1. Field of the Invention
The present invention relates generally to a communication system, and more particularly to a method for transmitting a Sounding Reference Signal (SRS) in a Long Term Evolution (LTE) communication system and an apparatus using the same.
2. Description of the Related Art
The 3rd Generation Partner Project (3GPP) standardization organization is working on a next generation of wireless communication standard, i.e., LTE. In a physical layer interface, LTE adopts Orthogonal Frequency Division Multiplexing (OFDM) technology, which is different from conventional Code Division Multiple Access (CDMA) technology. OFDMA is used in downlink and Single Carrier Frequency Division Multiple Access (SCFDMA) is used in uplink. The technology used in LTE is effective to resist multi-path propagation, with the adoption of frequency domain equalization reducing complexity of the conventional time domain equalization, and is more suitable for bandwidth high-speed data transmission.
From a point of view of air-interface, the LTE standard techniques can be divided into two categories: a Time Division Duplex (TDD) system and a Frequency Division Duplex (FDD) system. The LTE system supports variable bandwidths. And typical bandwidths include 1.4 MHz, 3 MHz, 5 MHz, 10 MHz, 15 MHz and 20 MHz, which can meet demands of different scenarios.
FIG. 1 illustrates a physical layer frame structure for a LTE FDD system in which a length of radio frame (101) is 10 ms, including ten equally sized radio sub-frames (102) of 1 ms length. Each radio sub-frame consists of two equally sized timeslots (103) of 0.5 ms length.
FIG. 2 illustrates a PHY layer frame structure for an LTE TDD system. As illustrated in FIG. 2, a length of radio frame (201) is 10 ms, and includes ten equally sized radio sub-frames (204) of length 1 ms. Each five continuous radio sub-frames makes up a half-frame (202) having a 5 ms length. Different from the LTE-FDD system, a second (211) and seventh (212) radio sub-frame in LTE-TDD radio frame are two special sub-frames. A length of the special sub-frame is 1 ms, including three special slots, indicating a Downlink Pilot Time Slot (DwPTS) (205 or 208), a Guard Period (GP) (206 or 209) and Uplink Pilot Time Slot (UpPTS) (207 or 210), respectively. The lengths of the three special slots are variable and may be defined differently by each system, but the total length is 1 ms. The length of the UpPTS can be 0, 1, or 2 SCFDMA symbols. If the length of the UpPTS is 2, the UpPTS is used to transmit the uplink Short Random Access CHannel (RACH) or Uplink SRS signal or both the Short RACH and the SRS signal. If the length of the UpPTS is 1, the UpPTS is used to transmit the uplink SRS signal. The other eight sub-frames, except the special two, respectively consist of two slots of length 0.5 ms.
In the LTE system, according to network scheduling, a User Equipment (UE) sends an SRS to evolved NodeB (eNodeB). According to an analysis result of the SRS signal, the eNodeB estimates a quality of channel that is used for transmitting SRS from UE to eNodeB and scheduling data according to frequency selective characteristics. Further, the eNodeB performs timing tracking for UE by analyzing the SRS signal and performs a close-loop power control. According to a current standardizing process, main conclusions for SRS transmission in LTE FDD system include the eNodeB broadcasting the SRS in a designated cell as needed and SRS being periodically transmitted in some sub-frame in a designated cell. A period is selected from {2, 5, 10, 20, 40, 80, 160, and 320} ms.
After the UE receives the SRS in the designated cell, the ODFM symbol resource occupied by the SRS is not used when transmitting uplink data. In order to perform the transmission of SRS, the UE should receive a user-designated SRS signal transmitted from the network. The signal informs the user of the OFDM symbol resource that is used to transmit SRS. Currently, there is no description for transmitting the SRS for a designated UE in a PHY layer specification, which is accomplished in LTE.
Currently, a basic idea in the standard for the user-designated SRS signaling is that the signaling includes three parts: Duration, Period, and Offset, in which, the duration can use 1 bit to indicate that just one snapshot or infinite. The period value is selected from {2, 5, 10, 20, 40, 80, 160, 320} ms.
In LTE FDD, the offset is a time between two transmission times of each OFDM symbol of the SRS from the beginning of the SRS period, and the basic unit is 1 ms. In LTE TDD, the definition of offset is different from that in LTE FDD. Since in LTE TDD, the SRS can be transmitted in the UpPTS or the other uplink sub-frame, the uplink sub-frame may be discontinuous and the UpPTS occupies two OFDM symbols at most, the offset is defined as an interval between an OFDM symbol position used to transmit SRS and an OFDM symbol position used to transmit the SRS until the period of SRS transmission starts. For example, if the SRS symbol position at the period beginning is defined as 0, a symbol position used to transmit SRS is 3, which means that the interval between the two symbols is 3. Consequently, there is at most 2 OFDM symbol positions can be used to transmit SRS.
The manner of transmitting the SRS in LTE TDD is mainly the same as that in LTE FDD. However the system structure of LTE TDD is different from of that in LTE FDD. A difference is that in LTE TDD, a half-frame of length 5 ms has both uplink sub-frame and downlink sub-frame, a number of uplink sub-frames and downlink sub-frames are configured by the network. In some configuration, a half-frame of length 5 ms at least has one uplink sub-frame (excluding the UPPTS).
According to a principle that only one SRS is transmitted in one uplink sub-frame, there is only one SRS transmission in every 5 ms, and the system cannot achieve the SRS transmission with a 2 ms period. Therefore, the performance of the SRS transmission by the UE is deteriorated in a fast Time-varying channel.
Based on the difference between the LTE TDD and LTE FDD, the current configuration of the 2 ms transmission period for the SRS in the LTE FDD cannot be used in the LTE TDD system.